This invention concerns blast hole drill bits and is especially concerned with a bit for forming longitudinal grooves in already drilled blast holes.
A study conducted for the United States Department of Transportation has concluded that longitudinal V-notches or grooves formed along the peripheral surfaces of predrilled blast holes aids in the control of crack propagation during blasting of the bore hole.
The study, entitled "Field Evaluation of Fracture Control in Tunnel Blasting," (Report No. UMTA-MA-06-0100-79-14) determined the effect of groove geometry on the crack initiation phase of controlled fracture. The study indicates that grooved blast holes may be a more efficient alternative to conventional drill and blast procedures. The notched holes require significantly less explosives to achieve the same results as fully charged conventional holes. This results in lower blast noise levels and less vibration. This is very beneficial, particularly in highly populated areas.
Conventional drill bits for drilling blast holes usually are comprised of a bit body having a forward working face. The forward working face has wear resistant compacts of the button, log cabin or other configuration, embedded therein and protruding out of the working face.
Tools for forming grooves in roof bolt holes are also known, such as shown and described in U.S. Pat. No. 3,960,222, granted to applicant corporation. Such tools do not readily lend themselves to forming longitudinal grooves in rock formations in which blast holes are drilled.
The cutting elements used in the referenced patent and in U.S. Pat. No. 2,879,973, also granted to applicant corporation, which discloses a percussion drill bit, are not designed to form a V-notch in the wall of a blast hole. U.S. Pat. No. 3,191,700, granted to applicant corporation, discloses a rotary percussion drill bit having cemented carbide cutting inserts with a V-shaped cutting edge. However, the included angle formed by the V-shaped cutting edge is too large and, therefore, would not lend itself to use in forming a sharp notch in a blast hole.